Electrolysers,particularly for chlorine-gas production



' Oct. 20, 1970 J, BAEKLUND ETAL ELECTROLYSERS, PARTICULARLY FORCHLORINE-GAS PRODUCTION Original Filed May 5, 1964 3 Sheets-Sheet l Oct.20, 1970 BAECKLUND EI'AL 3,535,223

ELECTROLYSERS, PARTICULARLY FOR CHLORINE-GAS PRODUCTION 3 Sheets-Sheet 2Original Filed May 5, 1964 Fig.4

.IIHII! Oct. 20, 1970 BAECKLUND EIAL 3,535,223

ELEGTROLYSERS, PARTICULARLY FOR CHLORINE-GAS PRODUCTION 5 Sheets-Sheet 3Original Filed May 5, 1964 n n UJJJJJJJJJ JJQ Fig.7-

United States Patent 3,535,223 ELECTROLYSERS. PARTICULARLY FORCHLORINE-GAS PRODUCTION Johannes Baecklund and Erik Reinhold Olson, Bro,

Timra, Sweden, assignors to Avesta Jernerks Aktiebolag, Avesta, SwedenContinuation of application Ser. No. 365,129, May 5, 1964. Thisapplication Oct. 6, 1969, Ser. No. 866,096 Claims priority, applicationSweden, May 6, 1963, 4,948/63 Int. Cl. B01k 3/00 US Cl. 204-275 9 ClaimsABSTRACT OF THE DISCLOSURE This invention pertains to a novelconstruction for electrolytic cells, and in particular to a novel formof anode that has a cavity through which the electrolyte may pass intothe inter-electrode space.

This is a continuation of application Ser. No. 365,129, filed May 5,1964, now abandoned.

For the electrolysis of aqueous solutions of alkali chlorides, it iscommon practice today to use graphite as the anode material. In spite ofthe fact that graphite is resistant to chemical attacking, and althoughits quality has been improved more and more, the anode blocks aresubject to a more or less rapid erosion in the course of theelectrolysis. This depends primarily on the oxidation of bothgraphitised and ungraphitised carbon which takes place due to anodicside reactions. This involves the drawback that bothelectrolyte andproducts will be contaminated by graphite sludge and carbon dioxide andthat the inter-electrode spacing will be increased.

In chlorine-alkali electrolysis using diaphragm cells, this meanscontamination of the chlorine gas by about 1% CO and certain organiccompounds formed from, inter alia, carbon, oxygen, hydrogen andchlorine, contamination of the alkali-salt solution mixture by extremelyminute graphite particles, choking of the diaphragm with sludge,increased voltage and frequent electrode replacements. All these factorsinvolve increased expenses. The most important detrimental factor,however, is the increased inter-electrode spacing which increases theenergy consumption.

In the chlorine-alkali production carried out in electrolysers operatingwith a horizontal mercury cathode, similar and additional drawbacks areencountered. In these cells, however, there is no diaphragm to capturethe main portion of the graphite slude, so that this sludge will have tobe carried away from the cell with the electrolyte. It tends to depositwithin the cell and to disturb the uniformity of the flowing mercurylayer. Larger graphite grains may precipitate into the amalgam layer,and in both cases the graphite residues cause the chlorine gas to becontaminated by hydrogen gas. To eliminate the increase of theinter-electrode spacing, the top covers of the mercury Cells must beconstructed so as to enable the anodes to be lowered at the same rate asthe anode erosion is proceeding. These arrangements and the labourinvolved in adjustments of the inter-electrode spacing increase the costof the equipment and products.

The above-mentioned drawbacks involved in the use of graphite anodeshave caused the technicians of these industries to desire a differentanode material. In the earliest period of chlorine-alkali electrolysis,platinum was used in many cases, but rather soon its price becameeconomically prohibitive to its continued use. Some years ago titaniumwas taken into use as a constructional material within thechlorine-alkali industry. Its extremely high resistance to wet chlorineand to chlorine-saturated ice salt solution very soon gave rise to theidea of using this metal as a carrier of a very thin layer of platinumand of constructing from these two materials electrodes to be used,primarily, for chlorine-alkali electrolysers.

The invention has for its object to provide an improved electrolyser,particularly for the production of chlorine gas, in which the anodes, atleast, are made of metal. Metallic electrodes, by suitable design, maybe made extraordinarily rigid while using a comparatively small amountof metal, and this rigidity in it's turn will enable shortinter-electrode spacings to be chosen, to thereby maintain the energyconsumption at a low level. However, short inter-electrode spacingsinvolve difficulties in respect of the possibility of ensuring a uniformsupply of fresh electrolyte to the narrow space between the anode andcathode. In accordance with the invention, this problem has been solvedby forming the metallic anodes with cavities for receiving freshelectrolyte and from which cavities the fresh electrolyte is passed oninto the interelectrode space through suitably distributed outletopenings.

The formation of the metallic anodes, in accordance with the invention,as distributing elements for fresh electrolyte Will cause the freshelectrolyte to be distributed uniformly over the entire surface of theanode, thereby minimizing the electrical resistance of the electrolytelayer between anode and cathode, and keeping the energy losses at a lowlevel. Further, the dwelling time of the electrolyte in the portion ofthe system where the electrolysis takes place will be as short aspossible, thereby reducing the effects of any undesirable sidereactions. During its passage through the hollow anode, which is heatedby the current losses, the electrolyte will be pre-heated, which is ofadvantage, and at the same time any dangerous local overheating of theanode is avoided due to the cooling effect of the electrolyte.

Where electrolysers having horizontal metal anodes are concerned, theanodes are preferably made in the form of shallow boxes in which thebottom wall forms the anode plate, providing in the bottom wall aplurality of outlet openings through which the fresh electrolytesupplied to each box separately will enter the space between theelectrodes. In electrolysers having vertically extending electrodes, thefresh-electrolyte receiving cavities in the metallic anodes may beconstituted by vertically extending electrolyte supply ductscommunicating through outlet openings with the inter-electrode space.

The invention will now be described more in detail in conjunction withthe accompanying drawings, in which:

FIGS. 1, 2 and 3 illustrate in top-plan view, side elevation and incross section along line IIIIII of FIG. 1, respectively, the inventionin its application to an electrolyser having a horizontally extendingmercury cathode;

FIG. 4 is a perspective view on a larger scale of one of the metallicanodes;

FIG. 5 is a cross section through a diaphragm cell for chlorine-alkalielectrolysis equipped with metallic anodes formed in accordance with theinvention;

FIGS. 6 and 7 show, in top-plan view with the topcover removed, and inlongitudinal section along line VII-VII in FIG. 6, respectively, anelectrolyser having vertically extending metallic electrodes.

In the embodiment shown in FIGS. 1 to 4, the electrolyser vessel is inthe form of an elongated trough 10 having a flat bottom 11 made of ironand walls 12 and 13 of ebonite-covered steel. The longer side walls 12have been reinforced mechanically by being formed as channel membersplaced on an edge. The cathode is formed of a layer of mercury 14flowing slowly along the bottom of the trough, the mercury beingsupplied at one end of the trough and leaving the latter through aliquid-seal at the opposite end. The metallic bottom 11 of the trough isconnected by a lead to the negative terminal of an electrical powersupply.

From the top edges of the electrically insulated side walls 12 of theelectrolyser vessel a plurality of metallic anodes 16 are suspended inspaced relation, these anodes being formed in accordance with theinvention. Each anode consists of a shallow, rectangular box made oftitanium and having flat end and side walls 17 and 18 and a corrugatedbottom wall 19. The end walls 17 are formed at their top edges withbent-out lugs 20 serving to support the anode box on the trough edges.At one end wall the length of the lug is extended to form a terminalstrip for connection to a bus bar 21 which, in its turn, is connected tothe positive terminal of the power supply. Numetal 22 designates aclose-fitting top cover for the electrolyser trough. The cover 22 isformed with an opening having secured therein a pipe socket 23 adaptedto be connected to an outlet pipe for the chlorine gas formed.

As clearly visible particularly in FIG. 4, the corrugations formed inthe bottom wall 19 of each anode box 16 extend in the longitudinaldirection of the electrolysis trough 10 and consist of depending ridges24 spaced by relatively narrow channels 25. The bottom surface of eachridge 24 is slightly curved downwards and is coated by a thin layer 26of platinum. The platinum plating is extended for a short distanceupwards over the vertically extending bottom surfaces defining thechannels. Extending along the top of the row of anode boxes 16 is anopen channel 27 for supplying fresh electrolyte. This channel is formeddirectly above each anode box with a bottom opening 28 through whichfresh electrolyte is continually supplied into the box, and in thebottom wall of the box a row of openings 29 is provided near one of theside walls, one opening in each ridge, and through said openings theelectrolyte is passed into the electrolysis trough 10 in a uniformpattern of distribution throughout its transverse width, whereby theelectrolyte will be uniformly distributed over the anode surface. Theelectrolyte supplied, after leaving the space between the electrodes,will flow slowly in the longitudinal direction of the trough through thepassageways formed adjacent each side wall, and will ultimately reach anoverflow at one end wall of the trough.

The respective anode boxes may be made in any desired size. In an actualcase their dimensions are as follows: length 70 cm., width 24 cm., anddepth 10 cm.

The electrolyte consists of an aqueous solution of alkali chloride. Whenapplying a suitable voltage between the anodes and the cathode, the ionsof alkali metal will migrate through the electrolyte to the mercurycathode 14 where they will amalgamate with the mercury. The chlorineions will migrate upwards to the active anode surfaces, i.e. to theplatinum-plated bottom surfaces of the ridges 24, where they will formminute bubbles of chlorine gas. Due to the static pressure of theelectrolyte and the curvature of these surfaces, the gas bubbles willmove rapidly over the surfaces and upwards into the channels where theywill form a more or less continuous gas cushion in the upper portion ofthe respective channels and will flow along the channels to the sides ofthe anode box where the chlorine gas will bubble up through theelectrolyte and leave the free upper liquid surface and enter the gascollecting space beneath the top cover 22. From this space the gas willbe passed out through the outlet 23 to be collected and disposed of inany suitable manner.

By corrugating the bottom wall 19 of the anode boxes 16 in the mannershown, a rigid horizontal metal anode has been provided, where only avery small portion of its total surface area is not electrically active,and where, in spite of this fact, no large and continuous gas bubbleswill form on the bottom surface of the electrode and detract from theeffectivity of the electrolyser. This has been effected by creating thegas collecting spaces uniformly distributed throughout the electrodesurface and represented by the impressed bottom channels 25. It isreadily possible to form the corrugations so as to cause 70 to of thehorizontally extending anode surface area to be electrolytically active.

In dimensioning the electrode boxes 16, it should be ensured, in thefirst place, that the box will be rigid with the active anode surfacesdisposed accurately in one common plane, thereby maintaining equaldistance between all parts of the anode surface and the plane mercurycathode level, and thus a substantially uniform current densitythroughout the anode surface. It is of particular importance to impartto the side walls 18 of the box a sufficient cross sectional area tocause the voltage drops to be negligible and to enable substantially thesame potential to be maintained in every point of the anode surface.

It is of extremely great importance to the useful service life of theanodes that the platinum layer be applied to the carrier metal, i.e.titanium, in a technically correct manner so as to cause the platinumlayer to adhere satisfactorily to the carrier metal, or backing, withoutany scaling-off tendency. It should be noted, however, that if, for somereason or other, the platinum layer should be subject to injury, anelectrically insulating oxide skin will form very rapidly on the exposedtitanium surface, and that such oxide skin will prevent any furthererosion of the titanium metal. Such a protective oxide skin, of course,will also form on all surfaces which are initially uncoated withplatinum.

Referring now to the electrolyser shown in FIG. 5, the trough 30containing the electrolyte (alkali chloride in aqueous solution) isassumed to be made of metal throughout. Mounted on the trough bottom area plurality of iron cathodes 31 disposed in side-by-side relation andspaced from each other by a certain clearance, and suspended above eachcathode from the top edges of the trough and with insulating strips 32interposed therebetween is an anode box 33 of the same design as thatshown in FIG. 4. A diaphragm 34, consisting, for instance, of Teflonfoil, is supported directly on the active top surfaces of the cathodes31. In the course of the electrolysis the chlorine ions migrate to thebottom surface of the bottom walls of the anode boxes 33 and thechlorine gas is passed through the channels in the box bottoms to thespace above the free upper surface of the electrolyte, from which spacethe chlorine gas is passed on to any suitable point of collection ordisposal. The presence of the gas collecting channels impressed in thebottom walls of the anode boxes, as before, will prevent any deleteriouscollection of chlorine gas beneath the horizontally extending activesurfaces. The ions of the alkali metal migrate through the diaphragm 34to the top faces of the cathodes 31 where the alkali metal immediatelyreacts with water while forming a hydrate which is dissolved in theelectrolyte, and hydrogen gas. The hydrogen gas is collected in adownwardly open channel 35 on the underside of the diaphragm 34 and iscarried away through this channel. In order to prevent any collection ofhydrogen gas on the bottom surface of the diaphragm 34, it is preferableto dispose the anodes and cathodes with their active surfaces slopingfrom one side wall of the electrolysis trough towards the opposite sidewall, as indicated in FIG. 5. Fresh electrolyte is continuously suppliedin a manner similar to that of the electrolyser shown in FIGS. 1 to 4,and at the same time an equal, amount of electrolyte contaminated withalkali hydrate is discharged from the space below the diaphragm.

FIGS. 6 and 7 diagrammatically show an electrolyser of a modifieddesign. The electrolysis trough 36 is made entirely of iron, andprojecting upwards from the trough bottom are vertically disposedplate-shaped iron cathodes 37 in uniformly spaced parallel relation anddistributed along the length of the trough. The anodes, being made oftitanium, are in the form of vertically extending, flattened tubes 38which are closed at their bottom ends where they terminate at a certainheight above the bottom of the electrolysis trough. As shown in thedrawing, between any two adjacent cathode plates 37 a plurality of suchtubes are disposed in laterally closely spaced interrelation, wherebythe tubes together form a substantially continuous plate-shaped anode.As an alternative, the anode may be formed by one single tube or by asmaller number of tubes. On their surfaces facing the cathodes 37 theanode tubes 38 are coated with a layer 39 of platinum, and are alsoformed with suitably spaced side openings. Fresh electrolyte iscontinually supplied to the trough through the tubular anodes 38provided with outlet openings, and spent electrolyte is continuallydischarged over an overflow, not shown, at one end wall of the trough.Owing to the vertical arrangement of anodes and cathodes, the chlorinegas will show no tendency of collecting at the platinum-plated activesurfaces of the anodes. The box-sectioned construction of the anodes andthe utilization of the interior of the box for supplying the electrolytethrough openings made in the box walls will ensure a uniform supply offresh electrolyte to all active electrode surfaces.

If it should be deemed to be desirable, the anodes may be fixed relativeto the adjacent cathodes by means of electrically insulating andchemically resistant spacers made of polytetrafluoroethylene, forexample. This would enable the assembly of a greater or smaller numberof electrode groups into compact packs or elements, thereby reducing thefloor area required for the installation.

The concept of supplying the electrolyte through vertically disposedanodes in the form of perforated tubes or boxes, of course, may beapplied not only to electrolysers having plate-shaped electrodes, butalso to electrolysers of the type Where the electrodes are constitutedby concentrically nested cylinders. In this case each anode may beformed with one single passageway, or with a plurality of passageways,for receiving and supplying the electrolyte.

Obviously, the invention may be applied to electrolysers having anodesmade of other chlorine-resistant metals than titanium, and havingcoatings of a metal other than platinum. Such carrier or backing metal",however, similarly to titanium, should be one adapted to exert an anodicbarrier effect, for instance by the formation of an electricallyinsulating surface skin as a result of the electrolysis. Such metalsare, for example, niobium (columbiurn), tantalum, tungsten andzirconium. As the elec trically conductive surface coating, besidesplatinum, also rhodium, iridium and palladium may come into question.However, at the present marketing prices, it would be preferable, as arule, to use titanium as the carrier or backing metal and platinum asthe electrically conductive surface coating.

Although the invention has been described hereinbefore in itsapplication to electrolysers for processing aqueous solutions of alkalichlorides, it could, of course, to equal advantage be applied to otherelectrolytical processes which can be carried out by the use of metallicelectrodes. As an example of such processes, the electrolyticalproduction of perborates may be mentioned, in which case no gasdevelopment is aimed at, but where a uniform supply of fresh electrolyteaccording to the present invention could, to advantage, be employed inthe electroplating field, as well. The particular electrode designconcerned is also of value in applications where the electrolyte or theresulting products are non-erosive so that no special requirements as tochemical resistance need be placed on the electrode metal.

What is claimed is:

1. An electrolytic cell which includes at least one anode, and

at least one cathode the improvement which comprises:

(a) at least one of said metallic anodes being a nonporous metallicanode,

(b) each metallic non-porous anode having bottom and side walls that areshaped to form a cavity that will create at least a limited reservoir ofa liquid upon the introduction of fresh electrolyte,

(c) each metallic non-porous anode having a liquid inlet opening intosaid cavity for the introduction of fresh electrolyte, to theelectrolytic cell, and

(d) each metallic non-porous anode being provided with at least oneliquid outlet for the electrolyte that is introduced into said cavity,said outlet communicating with the inter-electrode space between saidanode and said cathode.

2. An electrolytic cell as set forth in claim 1 having a plurality ofhorizontally extending metal anodes, said metal anodes being constructedin the form of shallow boxes, each box having a bottom wall that iscorrugated and forms the anode plate, the bottom wall of each box alsobeing provided with a plurality of outlet openings through which theelectrolyte received in the box will enter the space between theelectrodes. 3. An electrolytic cell as set forth in claim 2, in whichthe bottom wall of each box is in the form of a corrugated anode platehaving its corrugations extending longitudinally of the electrolyticcell, and said outlet openings are made in the ridge portions of thecorrugated anode plate that are closest to the cathode and suitablyclose to one of the side walls of the box extending transversely to thecorrugations.

4. An electrolytic cell according to claim 2 wherein a plurality ofcathodes are disposed in side-by-side relation beneath said metallicanodes, said cathodes being spaced from each other by a certainclearance.

5. An electrolytic cell according to claim 4 wherein a diaphragm issupported directly on the active top surfaces of said plurality ofcathodes.

6. An electrolytic cell according to claim 5 wherein said diaphragmcomprises polytetrafluoroethylene foil.

7. An electrolytic cell according to claim 5 wherein said diaphragm issloping and a channel is provided adjacent one side of the diaphragm forcollection of gas which forms below said diaphragm.

8. An electrolytic cell according to claim 1 wherein said metallicanodes are generally tubular in configuration, each metallic anode isdisposed between two cathodes, and the tubular walls of the anodescontain outlets which permit fresh liquid electrolyte introduced intothe cavity of each anode to pass through the anode into theinterelectrode space.

9. An electrolytic cell according to claim 8 wherein there are aplurality of anodes arranged vertically in sideby-side relationship andwherein each metallic anode has a generally rectangular cross section.

References Cited UNITED STATES PATENTS 1,003,456 9/1911 'Hazard-Flamand20426O 1,074,549 9/1913 Henkel et al 204260 1,575,627 3/1926 Heinze204-278 2,000,815 5/1935 Berl 204--9 2,273,795 2/1942 Heise et a1.2041.06 XR 2,643,223 6/1953 Notvest 204 3,103,473 9/1963 Juda 2041.06 XR3,310,482 3/1967 Bon et al. 204219 FOREIGN PATENTS 106,717 10/ 1898Germany. 708,023 4/ 1954 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner A. C. PRESCOTT, Assistant ExaminerUS. Cl. X.R.

